The present invention relates to the formation of a resist pattern on a substrate surface by a direct imaging process (i.e. without use of a phototool), and more particularly to the use of such direct imaging process in the fabrication of a printed circuit board.
A primed circuit board (PCB) fundamentally consists of a substrate made of dielectric material (such as epoxy, polyimide or the like, typically glass-reinforced) carrying a pattern of conductors on one or both of its faces. As used herein, PCBs are intended to include simple single- or double-sided PCBs; single- or double-sided innerlayer PCBs for use in fabricating multilayer PCBs; and multilayer PCBs per se.
Numerous techniques have been developed for forming the conductor pattern of PCBs, including subtractive techniques, additive techniques, and techniques involving both additive and subtractive aspects. Common to such methods is the reliance upon a patterned resist to define (either positively or negatively) selected surfaces of the board which will serve as the conductor pattern. Thus, for example, a dielectric substrate surface can be patterned with resist in the negative of the desired conductor pattern, followed by metal plating of the areas not covered by resist so as to provide the conductor pattern, followed by removal of the resist. In a more commonly employed method, the starting material is a dielectric having a metal layer (e.g., copper) coveting one or both of its faces, and over which metal layer is arranged a resist pattern in the positive of the desired circuitry pattern. The metal areas not protected by resist are etched away down to the dielectric substrate surface, whereupon subsequent removal of the resist reveals the desired conductor pattern. In another common technique, the metal-clad dielectric is patterned with a first resist in the negative of the desired conductor pattern; the non-resist covered areas are then built up with further metal; the so-metallized patterned areas then protected by a second resist in the same pattern; the first resist then removed and the metal previously thereunder etched away; and the second resist then removed to reveal the desired conductor pattern.
Apart from resists utilized in the foregoing manners (in which uses they typically are referred to as "primary" resists), resists also are utilized to provide a permanent coating (known as a "solder mask") over selected surface areas of a PCB so as to protect these areas from corrosion, oxidation, etc., and to protect them during subsequent soldering steps carried out on the PCB. Use of resist in this manner often is referred to as a "secondary" resist.
The provision of necessary resist patterns in fabricating PCBs is at present effected almost exclusively with photoresists. Photoresists are applied to surfaces in question in the form of a film, but not directly in the ultimately desired resist pattern. Rather, the composition of the photoresist is such that it undergoes photo (light)-initiated change when exposed to a radiation source of appropriate wavelength (typically, UV radiation). Thus, by selective imagewise exposure of the photoresist film to radiation, there is created in the film a latent image which can then be developed to provide the desired resist pattern.
In particular, photoresists are formulated of photo-sensitive compositions such that the photo-initiated change which occurs upon exposure to activating radiation is to render the exposed areas differentially more or differentially less soluble in a developer than unexposed areas. The solubility difference thereby permits areas of the photoresist film to be selectively removed with developer while leaving behind other areas which serve as the resist pattern.
Photoresists may be either "positive"-working or "negative"-working. A positive photoresist is one in which the exposure to UV light irradiation brings about photo-initiated reactions which cause the so-exposed areas to become differentially more soluble in developer than the unexposed areas. These reactions may, e.g., be in the nature of a photo-induced depolymerization of polymeric resin materials in the composition through breaking of chemical bonds in the polymer chain; or photo-induced change in the structure of a compound present in the composition which enables the entirety of the composition, including its resin components, to become more soluble in developer; or other like reactions. A negative photoresist, on the other hand, is one in which the exposure to UV light irradiation brings about photo-initiated reactions which cause the so-exposed areas to become differentially less soluble in developer than unexposed areas, such as may occur through utilization of a composition which undergoes photo-initiated polymerization, additional polymerization or cross-linking, such as by free radical mechanisms. Accordingly, when using a positive photoresist, those portions exposed to UV light irradiation are selectively removed by developer, while in a negative photoresist, the developer selectively removes those portions which were not exposed to UV light irradiation.
For providing the necessary photoresist film used in producing primary resists, generally any of three types of photoresists can be employed. One type is known as a dry film, which is provided in the form of a pre-existing film which is then laminated to the substrate surface. See, e.g., U.S. Pat. No. 3,469,982. Dry films, which in practice are exclusively negative photoresists, are overwhelmingly preferred today in mass production of PCBs.
Another type of photoresist film is that obtained from a liquid photoresist composition (which may be a positive or negative photoresist) which is applied to the substrate as a film by roller coating, dipping, spinning or the like, followed by drying of the film to eliminate all or most of the solubilizing solvents used therein.
A third type of photoresist film is that obtained by electrophoretic deposition onto metal substrate surfaces, i.e., using a liquid photoresist composition which is formulated so as to be electrophoretically depositable. See, e.g., U.S. Pat. Nos. 4,592,816; 4,751,172; and 5,004,672.
For the provision of solder masks, the photoresists employed are predominantly liquid, negative photoresist compositions applied as a film by roller coating, curtain coating or the like. A typical process for producing a solder mask can be found in U.S. Pat. No. 4,789,620, the teachings of which are incorporated herein by reference.
The imagewise exposure of the photoresist film, i.e., the exposure of only selected portions of it to UV light irradiation to produce the desired latent image, is predominantly effected through use of an appropriately patterned mask (phototool) which permits passage of UV light there through, to the film, only in the desired locations. The difficulty with such phototools is that their preparation is very expensive and time-consuming. Moreover, the use of phototools for selective imaging imposes limits on the achievable resolution of the conductor pattern for a variety of reasons.
In recent years, attention has been directed to so-called "direct imaging" techniques for photoresists. In these direct imaging methods, the exposure of only selected areas of the photoresist film to the activating radiation needed to bring about the required photoinitiated changes in the film composition does not utilize a radiation source directed through a patterned phototool, but rather employs a suitably focused beam of such radiation (such as with a laser of appropriate wavelength light) which directly scans the film in a predetermined (computer-controlled) desired tracking pattern. See, e.g., U.S. Pat. No. 4,724,465 the teachings of which are incorporated herein by reference; Kuchta, A. D., "Technological Requirements for Direct Imaging of Photoresists", Technical Paper No. A 8/1, Printed Circuit Word Convention 5 (June 1990); Meier, K., "Laser Direct Imaging In High Definition Image Transfer Processes", Technical Paper No. A 8/2, Printed Circuit Word Convention 5 (June 1990). While such techniques offer potential advantages in resolution capability and avoidance of defects sometimes caused by imperfect phototool artwork and/or by operator handling of phototools, those developed to date are too slow for mass production; require expensive photoresists; and rely upon lasers which have a short life unsuited for industrial use.
In other areas direct imaging has been employed in the production of printing plates. See U.S. Pat. No. 5,310,869 (May 10, 1994), the teachings of which are incorporated herein by reference, where the printing plates are coated with a two-component silicone based formulation. The two components are combined in varying proportions with a cross linking agent to produce compositions with varying viscosities and dispersibilities. In addition reference is made to U.S. Pat. No. 5,339,737 (Aug. 23, 1994) where printing plates are produced using an ablatable material. The printing plate there is composed of one or more layers having two functions, absorption ofinfrared radiation and interaction with ink or ink-adhesive fluid.